1. Field of the Invention
The present invention relates generally to processing of encrypted signals and, in particular, to processing of such signals which include speech information at a receiver after occurrence of a transmission error.
2. Description of the Prior Art
When a data stream which includes speech information is encrypted using a one-bit cipher feedback (also sometimes known as cipher text auto key or CTAK) technique, the occurrence of a single bit error in the transmission facility connecting the transmitter and receiver is expanded into an n+1 bit error burst after decryption, where n is the length of the encryption key register. This burst causes an undesirable audible "click" which can be quite annoying. To avoid the problem, an output feedback (key auto key) encryption scheme (for example, as described in Federal Information Processing Standards, Publication 81, "DES Modes of Operation") can instead be used in order to limit the burst length. However, this requires external cryptographic synchronization. Alternatively, retransmission of the original information can be attempted, but this increases processing delay. With yet another approach, redundant information can be encoded with the speech signal to enable easy detection and correction of the error. This, unfortunately, adds complexity and thus cost to the system and reduces the available data rate for nonredundant information.